Bt has to be eaten to cause mortality. The Bt
toxin dissolve in the high pH insect gut and become active. The toxins
then attack the gut cells of the insect, punching holes in the lining.
The Bt spores spills out of the gut and germinate in the insect
causing death within a couple days.

Even though the toxin does not kill the insect immediately,
treated plant parts will not be damaged because the insect stops feeding
within hours. Bt spores do not spread to other insects or cause
disease outbreaks on their own.

1. Insect eats Bt crystals
and spores.

2. The toxin binds to specific receptors in the gut and the insects
stops eating.

3. The crystals cause the gut wall to break down, allowing spores
and normal gut bacteria to enter the body.

4. The insect dies as spores and gut bacteria proliferate in the
body.

Bt action is very specific. Different strains
of Bt are specific to different receptors in insect gut wall.
Bt toxicity depends on recognizing receptors, damage to the gut
by the toxin occurs upon binding to a receptor. Each insect species possesses
different types of receptors that will match only certain toxin proteins,
like a lock to a key.

It is because of this that farmers have to be careful
to match the target pest species with a particular Bt toxin protein
which is specific for that insect. This also helps the benifical insects
because they will usually not be harmed by that particular strain of Bt.